The hypothesis of this proposal is that the lactotrope-specific and Ras-regulated expression of PRL is governed by the selective Ets-1/Pit-1 protein partnership, which in turn is dictated by a specific protein- protein interaction domain and organization of binding sites in the composite RRE. The four specific aims are to: 1) map the precise amino acids of Ets-1 and Pit-1 required for their functional and physical interaction; 2) define the distance and orientation requirements for the two protein binding sites in the composite elements; 3) identify the Ets factors in GH4 cells that specifically recognize the Ets site in the RRE; and 4) elucidate the role of Ets factors in endogenous PRL gene expression.